Automobile windshields ordinarily consist of two sheets of glass and an interposed layer of a thermoplastic material, such as polyvinyl butyral, bonded to one another by heat and pressure to form a composite unit. The bonding is usually accomplished in a two-step operation by first subjecting the assembled laminations to a preliminary pressing to remove the air from between the laminations and effect superficial bonding thereof, and then to a final heating and pressing operation in an autoclave filled with oil or air to completely bond the laminations together.
Different methods have been employed for initially removing the air from between the laminations. One of these consists in passing the glass-plastic assembly between squeeze rollers to force the entrapped air from between the laminations. Another is to provide a flexible channel shaped member in engagement with the marginal edge portion only of the assembly, in spaced relation to at least part of the periphery thereof, to provide a conduit through which the air can be withdrawn. Still another is to place the assembly in a flexible plastic bag, seal the bag, and apply a vacuum thereto to withdraw air from within the bag as well as from between the laminations. These bags are usually formed of films or sheets of polyvinyl alcohol.